REVIEW ON 3D FABRICATION AT NANOSCALE

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Recent years have witnessed a rapid expansion of research on advanced nanotechnologies for 3D nanofabrication.In top-down approaches, various nanolithography techniques, such as optical lithography, electron-beam lithography (EBL), interference lithography (IL), direct laser writing (DLW), nanoimprint lithography (NIL), and 3D nanoprinting methods (such as focused electron beam (FEB)-induced deposition, focused ion beam (FIB)-induced deposition, and direct ink writing (DIW)) have been employed to create a variety of 3D nanogeometries [13].In bottom-up approaches, selfassembly has been extensively used to fabricate versatile 3D geometries [14].Each of these techniques has demonstrated distinct advantages and revealed enormous potential for the fabrication of various classes of nanostructures and devices.Nevertheless, none provides a complete solution to the challenge of 3D nanofabrication, due to various combinations of disadvantages that include limited flexibility in the structure geometries, slow speeds, applicability only to relatively small areas, complex experimental setups, and uncertain yields and defect densities [15].This review provides a comprehensive overview of advanced fabrication technologies and processes currently available to generate 3D nanostructures and discusses their general advantages, existing challenges, and future prospects.

Abstract:
Among the different nanostructures that have been demonstrated as promising materials for various applications, three-dimensional (3D) nanostructures have attracted significant attention as building blocks for constructing highperformance nanodevices because of their unusual mechanical, electrical, thermal, optical, and magnetic properties arising from their novel size effects and abundant active catalytic/reactive sites due to the high specific surface area.Considerable research efforts have been devoted to designing, fabricating, and evaluating 3D nanostructures for applications, including structural composites, electronics, photonics, biomedical engineering, and energy.This review provides an overview of the nanofabrication strategies that have been developed to fabricate 3D functional architectures with exquisite control over their morphology at the nanoscale.The pros and cons of the typical synthetic methods and experimental protocols are reviewed and outlined.Future challenges of fabrication of 3D nanostructured materials are also discussed to further advance current nanoscience and nanotechnology.

Nanolithography
Lithography is a process used to generate nanoscale structures on substrates by first creating a pattern in the resist that covered the substrate and then transferred it to the substrate by etching or lift-off [16].Diverse lithographic patterning techniques, such as optical lithography, IL, EBL, DLW, and NIL have been widely employed to fabricate functional 3D nanostructures and systems [17].
Optical lithography (also known as photolithography) such as ultraviolet (UV) lithography and X-ray lithography are maskrequired semiconductor device fabrication methods used to create 2D and 3D on the surfaces of semiconductor materials.These two methods first use UV light or X-rays to transfer a geometric pattern from a mask to a light-sensitive chemical photoresist on the substrate.A series of chemical treatments then is used to etch the exposure pattern into the material or enables deposition of a new material in the desired pattern upon the material underneath the photoresist.The process for these two methods generally contains several steps (Fig. 4) [18].Optical lithography has been successfully employed to create 2D nanopattern layers.However, 3D nanofabrication needs to be achieved by stacking those layers.A set of different masks is required to form each of those different layers through overlay alignment.Furthermore, precise optical systems with vertical and horizontal steppers are required for repositioning and registration.Therefore, the overall procedure is tedious, expensive, and difficult to master [19,20].EBL, DLW, and IL are maskless lithography methods following a process similar to mask-required lithography, except that patterns are transferred onto substrates without utilizing an intermediate mask [13].EBL is one of the most important nanofabrication methods and is generally used by directly illuminating a high-voltage electron beam onto an electron-sensitive resist to create desired patterns down to a few nanometers [Fig.5(a)].However, EBL is also a 2D method [19,21].There are two strategies to apply EBL to 3D nanofabrication.One is the complex and time-consuming layer-by-layer stacking.The other one is using conformable elastomeric phase masks [22].DLW, also known as multiphoton lithography or direct laser lithography, is a laser-assisted fabrication technique performed by scanning a tightly focused laser beam according to designed patterns into a photosensitive material (photoresist) to form 3D structures with complex shapes and self-supporting features at resolutions down to tens of nanometers [Fig.5(b)] [23].3D nanostructures formed in this way rely on polymerization of specialized polymers via two-or multiphoton absorption.The formed photoresist with 3D nanopatterning then serves as a sacrificial template for forming structures in metals, ceramics, and other materials through conformal deposition and selective removal of the polymer [24].Currently, UV, nanosecond pulsed, excimer, and Nd:YAG are the most commonly used lasers.However, shorter pulse lasers, such as picosecond and femtosecond lasers, are also finding application as precision writing tools [25,26].Although DLW also shares the drawbacks of being time-consuming and having low throughput and high cost, it has been intensively exploited to fabricate various arbitrary 3D nanostructures (Fig. 6) [27].IL, also known as holographic lithography, utilizes the interference of coherent beams of lights to create 1D, 2D, and 3D periodic patterns (Fig. 7) [28,29].IL can create 3D nanoscale features without the use of complex optical systems, photomasks or conventional layer-by-layer stacking processes.IL provides access to various 3D nanostructures by altering the number of interfering beams, wave vectors, and phases.It allows rapid creation of large 3D nanostructures [30,31].There are two broad approaches currently employed to yield 3D interference patterns: phase mask IL and multibeam IL [31,32].However, IL suffers from index mismatching between the polymer and substrate, limited availability of space to arrange multiple beams, and registration problems.Solutions such as the use of phase shift mask and translational stages and multiple exposures have been employed to address these challenges [30].Researchers also explore the possibility to integrate multibeam IL and two-photon/multiphoton lithography to enable fast production of 3D structures with controlled and functional elements [27].
NIL relies on direct mechanical deformation of imprint resist and subsequent processes to create patterns [33].In the NIL process, a prefabricated mold containing an inverse of the desired patterns is first pressed onto a resist-coated substrate to replicate the patterns via mechanical deformation.Then a similar pattern transfer process, including etching and lift-off, can be used to transfer the pattern in resist onto the substrate [34].In terms of resist curing, there are two fundamental types of processes: thermal NIL and UV NIL (Fig. 8) [35].Since NIL replicates the nanopattern of the mold regardless of the diffraction limit set by light or beam scattering factors as observed in conventional nanolithography methods, it can achieve sufficiently high productivity and patterning resolution at low cost [36].A large volume of studies exist in the literature that have explored NIL-based technology for creating 3D nanoscale structures (Fig. 9) using various promising materials [37][38][39][40][41].The NIL process carries simplicity, low cost and high throughput.
Besides, the roll-to-roll-based NIL process shows a highly promising future to be implemented as a full-scale production process for various applications due to their high throughput and wide-area patterning capability [42].
Comparative analysis of various nanolithography methods is provided in Table 1.

3D Nanoprinting Techniques
3D printing is an additive manufacturing method to produce high-aspect-ratio 3D structures in a layer-by-layer manner without any additional process steps [43].Recent efforts on nanoscale 3D printing have provided an innovative series of tools for direct fabrication of 3D nanostructures.Some of the mentioned techniques directly transferring nanoparticles to the substrate include focused electron-beam-induced deposition, focused ion-beam-induced deposition, and DIW [44,45].
Focused electron-beam-induced deposition (FEBID) is an additive, direct-write approach for synthesis of 3D architectures on complex substrate topographies without the need of masks, resists, etching, or lift-off processes and is made possible by the scanning or transmission electron microscope (SEM/TEM) platform [45].In FEBID, injected gaseous precursor molecules are absorbed onto the sample surface and then dissociated by incident energetic electrons, leading to the formation of a solid deposit (Fig. 10) [46].The composition and the shortrange atomic order of the condensed molecular aggregate is a function of both the concentration of precursor molecules and the electron beam parameters such as beam accelerating voltage, current density, and dose.FEBID allows fabrication of highly complicated 3D geometries with nanometer resolution on morphologically challenging substrates as well as flexibility in structural design and material choice, as the adsorption of the precursor gas is not limited to flat surfaces and the position/ movement of the electron beam can be easily controlled in a well-defined manner [47,48].One main issue of FEBID is low speed and throughput when performed on single-electron beam machines.However, multibeam FEBID with 196 beams in one microscope was demonstrated, as well as FEB and FIB microscopes with >10 5 individually addressed beams, would allow boosting the overall speed accordingly (for 1000 beams) [49].The other main issue of FEBID is the existence of chemical impurities due to incompletely dissociated precursor molecules, leading to incorporation of unwanted fragments inside deposit, often containing C and O [50].Various post-growth purification strategies have been developed to improve the deposit purity.Due to these advantages of FEBID, in conjunction with the ever-growing reservoir of suitable precursors for a wide range of material properties of the deposits and the development of purification strategies and advanced gas injection systems (GISs), FEBID offers a roadmap to high-quality, multicomponent   Precursor molecules (here: organometallic complex; blue, metal; green, organic ligands) are supplied by a gasinjection system and physisorb (1) on the surface.Surface diffusion (2), thermally induced desorption (3) and electron-stimulated desorption (3') take place.Within the focus of the electron beam, adsorbed precursor molecules are (partly) dissociated followed by desorption of volatile organic ligands (4).Upper right: For pattern definition, the electron beam is moved in a raster fashion (here, serpentine) over the surface and settles on each dwell point for a specified dwell time.After one raster sequence is completed, the process is repeated until a predefined number of repeated loops is reached.(Reproduced from [46].CC BY 2.0.).nanomaterials [48].For example, Fig. 11 shows 3D nanoobjects fabricated via FEBID [45].
Focused ion-beam-induced deposition (FIBID) is a direct-write 3D nanofabrication technique similar to FEBID.The difference is that instead of using a focused electron beam as the writing tool, FIBID uses a closely focused beam of positively charged ions to direct write 3D nano-objects [48].During the FIBID process, the FIB breaks the gas precursor molecules coming from the GIS and leaves a deposit on the substrate, acting as a nucleation site for the nanostructure growth FEBID, substantial beam-induced damage on the substrate (amorphization, implantation, defects, etc.) has been observed for FIBID, which can be detrimental in some cases [56].Despite this, such "undesirable" defects have recently been utilized to develop a novel nanofabrication method (i.e., FIB nano-kirigami) to manufacture functional nanostructures and devices [48].
DIW is an extrusion-based printing technique to create arbitrary 3D shapes through the layer-by-layer deposition of printable inks.During the printing process, the ink with appropriate rheological properties is extruded from a fine nozzle and deposited on a substrate to form preprogrammed architecture For continuous filament writing (such as robotic deposition, pen writing, and fused deposition), the inks are continuously extruding from a nozzle and the filament element is patterned on a fixed platform to construct the objects.Droplet-based writing (such as ink jet printing, and hot melt printing) allows the ink to inject drop-by-drop on demand.Among liquid ink-based DIW techniques, fountain pen and dip-pen writing has been utilized to generate nanopatterns straight to the substrates with a variety of ink options [44].Aerosol ink-based DIW is an emerging contactless direct write approach for the production of arbitrary 3D structures using functional nanomaterial inks.In aerosol ink-based DIW, a functional ink is aerosolized and deposited to the substrate by a carrier gas [Fig.12(c)] [59,60].All DLW technologies offer the materials flexibility, high scalability, low cost, and ability to construct complex shape and multimaterial structures by using multiple extrusion nozzles.Diverse ink-materials including ceramics, metal alloys, polymers, food, biomaterials, colloidal gels, hydrogels, and electronically functional materials have been used for 3D printing [61].In contrast to liquid ink-based DIW, aerosol ink-based DIW allows the patterning of more complex surfaces and the use of 3D nonplanar substrates [62].Despite these potential advantages, some issues like structural distortion and defects at corner for liquid ink-based DIW and overspray for aerosol ink-based DIW still need to be faced [63].
Comparative analysis of various 3D nanoprinting methods is provided in Table 2.

Nanomachining
Nanomachining is a subtractive manufacturing method to produce structures with nanometer accuracy by removal of materials.Nanomachining technologies such as laser beam machining (LBM), tip-based nanomachining (TBN), and FIB milling have been explored for fabrication of complex 3D nanostructures made of different materials [64][65][66].
LBM is a noncontact machining technology that uses highintensity laser beams to cut or ablate materials (Fig. 13) [67].High frequency of monochromatic light falls on the surface, heats, melts, and vaporizes the materials.LBM is a versatile process that can shape a broad range of materials such as And different types of laser machining systems have been applied for these varieties of materials where complex shapes demand precise, fast, and force-free processing [68,69].For example, a CO 2 -based laser beam has been attempted to analyze the effects of process parameters on the surface performance of the titanium alloy [70].The femtosecond laser has been studied to fabricate high hardness microarray diamond tools [68].The picosecond laser has been used to produce smooth, deep and defect-free cuts of desired geometries on alumina ceramics with a high degree of precision [71].The nanosecond laser has been utilized to ablate SiCp/AA2024 composites with increased ablation depth and rate [72].These developed laser systems have been utilized for fabricating engineered net-shaping material systems for applications including bio-inspired architectured materials, interlocking sutures, topologically interlocked ceramics, semiconductors, electronics, surface treatments, multilayer ceramic packaging for microelectronics, scribing engineering, and laser surface cleaning [71].In general, LBM is a powerful machining method for cutting complex profiles and drilling holes in wide range of workpiece materials.Machining accuracy and surface quality depend on the process parameters and the composition and initial surface finish.However, the main disadvantages of this process are low energy efficiency from a production rate point of view and converging diverging shape of beam profile from the quality and accuracy point, low resolution limited to micrometers, and surface damage [68].
TBN has emerged from the well-established scanning probe microscopy (SPM) platform to manufacture nanointegrated systems of the second and third generations.In TBN, a nanoscale tip is brought into close proximity or contact with a substrate to modify the surface using thermal, mechanical, or electrical fields.In particular, it delivers capabilities for higher resolution (< 20 nm) manufacturing with process flexibility, integrated processing, assembly, metrology, and visualization in a cluster tool through the use of different nanoscale tool tips.TBN techniques can be categorized by the instrumentation platform to be scanning tunneling microscopy (STM), atomic force microscopy (AFM), and nanoindenter-based TBN [73,74].
In AFM tip-based TBN method, the AFM probe tip is employed as a nanocutting tool in the machining process, as shown in Fig. 14 [75].Currently, the AFM tip-based TBN method is known to be a low-cost, simple, highly accurate, and flexible method, and is widely employed in many research fields to machine nanoscale features [76].For example, AFM-based 3D nanofabrication assisted using ultrasonic vibration has been reported to fabricate a set of 3D nanostructures on polymethyl methacrylate (PMMA) samples [73].A modified AFM TBN SEM uses a focused beam of electrons to image the sample.However, FIB uses a finely focused beam of ions (usually Ga + ) that can be operated at high beam currents for site-specific milling or at low beam currents for imaging.FIB can also be incorporated in a system with both SEM and ion beam columns, which is called dual-beam (also called two-beam or multibeam) platforms.Dual-beam platforms, additionally equipped with GISs and secondary electron (SE) detectors, can serve as multifunctional tools for the purpose of milling the sample surface and as a method of imaging (Fig. 15) [78].During the process, the ion beam locally scans the surface, digging the targeted area.The sputtering rate is ruled by the energy of the beam, which hits and locally removes the substrate atoms.
During the sputtering removal, the ions are implanted into the sample.FIB milling can fabricate complex microfeatures process with enhanced throughput has been developed by combining fast reciprocating motions of a shear-type piezoelectric actuator and linear displacements of the AFM stage [77].To date, nanodots, lines/grooves, 2D/3D structures, and even nanostructures on curved surfaces have already been successfully obtained on different materials using the AFM TBN method.Despite these advantages, the AFM TBN method is not yet considered a serious competitor for nanoscale manufacturing.This is partly due to its small material removal rate and large setpoint force, which results in high tip wear rate, slow machining speed, and low throughput [77].
The FIB is a multifunctional tool that can inspect, cut, deposit, and touch a sample with nanoscale precision.It operates using a principle similar to a scanning electron microscope (SEM).on nearly all workpiece materials with high surface quality and dimensional accuracy because of ultralow ion scattering effect.At present, FIB milling has become the method of choice in various applications, such as the circuit editing of semiconductor devices, cross-sectional materials analysis, and transmission electron microscopy samples preparation.FIB milling is also employed to fabricate high-quality and highprecision components for optical, mechanical, thermofluidic, and biochemical applications [79,80].However, FIB milling is currently still limited to a micromachining method.The other fundamental feature that has so far hindered the widespread use of FIB milling in nanophotonics is the low purity level of the fabricated nanostructures [64].During the milling process, the impurities introduced by the Ga + implantations and redeposition of the sputtered atoms can alter the fabricated nanostructure, damaging the substrate [48,56].
Comparative analysis of various nanomachining methods is provided in Table 3.

Bottom-Up Techniques
Self-assembly is an important bottom-up method to fabricate nanostructures.Self-assembly is a process in which the individual components such as atoms, molecules, or a collection of molecules organize themselves into ordered, functional ensembles without external intervention [81].In the self-assembly process, various noncovalent intermolecular interactions such as van der Waals interaction, hydrophobic interaction, π-π stacking, hydrogen bonding, ion pairing, cation-π, anion-π, dipole-dipole, halogen bonding, and metal coordination interaction between disordered building blocks have been used to drive the system toward the formation of more ordered (or more organized) nanostructured systems (Fig. 16) [82].
Although most of the self-assembly is performed spontaneously, direct self-assembly is a more useful and controllable way to form 3D nanostructures with a more large-scale ordered and hierarchical pattern.Direct self-assembly is a self-assembly process driven by external factors.Different force fields such as gravity, centrifugal force, or electrostatic force, magnetic field, capillary force, light, ultrasound, and templates have been used to trigger or direct self-assembly [27,82].A great variety of materials, including colloids, crystalline materials, block copolymers, peptides, and deoxyribonucleic acid (DNA) have been self-assembled into ordered nanostructures [81,82].Metal oxide-based nanostructures, DNA-based nanostructures, and polymer-based nanostructures fabricated by self-assembly have found widespread applications in diverse fields [83].For example, a 3D microstructure that contains in situ growth of the Ni(OH) 2 nanowalls at specific locations has been fabricated for use as wafer-scaled miniaturized gas sensors (Fig. 17), and 3D flower-like hierarchical nitrogen-doped and carbonsensitized Nb 2 O 5 (N-NBO/C) nanostructures have been fabricated for solar energy transformation and environmental remediation (Fig. 18) [84,85].Despite this, the structural control over a larger scale range, mechanical properties of the selfassembly materials, the combination of different molecules into advances have been achieved, as it is still in the early stages, several important issues, such as limited resolution, low throughput, high cost, limited flexibility in material choice, and 3D structure shape, still remain.Researchers must put more efforts into systematically developing 3D nanomanufacturing theory.From the perspective of 3D fabrication at nanoscale, future research can be conducted at an even smaller scale (atomic or close-to-atomic).To benefit from 3D nanostructured materials' advantages and to address their challenges, more intensive research needs to be conducted to develop ultraprecise nanofabrication methods, especially aimed at massive fabrication of 3D nanostructures at nanoscale resolution and low cost with excellent repeatability, controllability, and great flexibility.Moreover, the material behavior is critically important for 3D fabrication at nanoscale.New high-resolution testing method should also be developed to comprehensively evaluate the 3D nanomanufacturing quality.Integrating is also an important trend (such as AFM TBN and dual-beam platforms) in developing 3D nanostructures.With the integrating established 3D nanomanufacturing methods, massive fabrication of highresolution nanostructures will be realized.
a complex system in a cooperative or syndetic way remains a challenge [82].

Conclusions and Future Outlook
This study mainly surveyed the recent advancement in synthesis and fabrication for 3D nanostructured materials and summarized the general advantages as well as the existing challenges of each technique    [ [55] Belianinov, A., et al. (2020).Direct write of 3D nanoscale mesh objects with platinum precursor via focused helium ion beam induced deposition.Micromachines, 11 (5), 527.

Figure 10 .
Figure10.Illustration of FEBID.Precursor molecules (here: organometallic complex; blue, metal; green, organic ligands) are supplied by a gasinjection system and physisorb (1) on the surface.Surface diffusion (2), thermally induced desorption (3) and electron-stimulated desorption (3') take place.Within the focus of the electron beam, adsorbed precursor molecules are (partly) dissociated followed by desorption of volatile organic ligands (4).Upper right: For pattern definition, the electron beam is moved in a raster fashion (here, serpentine) over the surface and settles on each dwell point for a specified dwell time.After one raster sequence is completed, the process is repeated until a predefined number of repeated loops is reached.(Reproduced from[46].CC BY 2.0.).

Figure 15 .
Figure 15.(a) Illustration of the specimen chamber of a dual-beam platform; (b) illustration of serial slicing and imaging application of dual-beam platforms.An ion beam is used for creating cross sections, while an electron beam allows for monitoring and imaging of the sliced regions.(c) Nanostructuring, nanofabrication, and maskless ion lithography examples performed by dual-beam instruments (Reproduced from [78].CC BY 3.0.).

Figure 17 .
Figure 17.(a) Schematic of fabricating wafer-scale miniaturized gas sensors; (b) dynamic response of eight gas sensors to different H 2 S concentrations; (c) response of eight gas sensors toward different gases; the concentration is 5 ppm (Reproduced from [84].CC BY 4.0.).

Table 1 .
Comparative study of different nanolithography methods.

Table 2 .
Comparative study of different 3D nanoprinting methods metals, polymers, glasses, ceramics, and composite materials.